Solid State Precipitation: Session II
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Phase Transformations Committee
Program Organizers: Seth Imhoff, Los Alamos National Laboratory; Robert Hackenberg, Los Alamos National Laboratory; Gregory Thompson, University of Alabama

Thursday 8:30 AM
March 2, 2017
Room: 25A
Location: San Diego Convention Ctr

Session Chair: Seth Imhoff, Los Alamos National Laboratory

8:30 AM  Invited
Self-organization by Strain Accommodation in the Formation of Long-range Stacking Order Structure in Mg-RE-TM Alloys: Tadashi Furuhara1; Xinfu Gu1; 1Tohoku University
    Long range periodic stacking ordered (LPSO) structures in magnesium alloy consist of regular arrangements of four layer high fcc structural units separated by several layers of the Mg basal plane. The diplacive nature in hcp->fcc structural transformatio involves large transformation strain. This strain can be significantly accommodated by the combination of the structure units with different shears. In addition, the possible lowest elastic energy configuration in both R and H type of LPSO structure is obtained. For H type LPSO structure, the opposite shear directions (or Burgers vectors of the partial dislocations) are operated in nearest structural units, while all three types of shear directions are operated successively in the nearest structural units for R type LPSO structure. Cs-corrected HAADF STEM observation has confirmed that the lowerst energy configuation is actually formed for the 14H LPSO structure in a Mg-Zn-Gd alloy.

9:00 AM  
Effects of Clustering and Trace Elements on Precipitation Hardening of Al-Mg-Si Alloys: Stefan Pogatscher1; Marion Werinos1; Peter Uggowitzer2; 1Montanuniversitaet Leoben; 2ETH Zürich
    Although Al-Mg-Si alloys are the most frequently used group of precipitation hardened aluminum alloys, early stages of aging are still far from being fully understood. Here we discuss the basic principles of low temperature clustering, its implications on precipitation reactions during artificial aging, and the effect of trace elements and suggest therefrom an improved picture of the underlying mechanisms. Hardness testing, differential scanning calorimetry, atom probe tomography and simulations were used to study clustering at different low temperatures from very short to very long times and subsequent artificial aging for conventional and trace element added alloys. Moreover, it will be shown which physical pre-requisites need to be fulfilled to examine a “diffusion on demand” concept to modify clustering and precipitation kinetics in Al-Mg-Si alloys by orders of magnitude via trace elements.

9:20 AM  Invited
Clustering and Precipitation in Al-Cu-Li Alloys: Influence of Minor Solute Additions on the Competition between Kinetic Paths: Alexis Deschamps1; Frederic De Geuser1; Eva Gumbmann1; Rosen Ivanov1; Christophe Sigli2; 1Grenoble Institute of Technology; 2Constellium Technology Centre
    Al-Cu-Li alloys developed for aerospace applications generally contain minor solute additions. These solute additions interact in a complex way with processing parameters (pre-deformation, ageing temperature, …) and change profoundly the kinetics path followed by the alloy towards the end distribution of strengthening precipitates. Their presence acts on a much broader scope that simply modifying the distribution of equilibrium phases, and involves interaction with excess vacancies, formation of clusters and precursor phases, and segregation at the interface boundaries. We will address the influence of small additions of Mg (and to a lesser extent of Ag and Zn) on clustering during natural ageing and on precipitation during artificial ageing. Using a combination of advanced characterization tools at the atomic scale (STEM-HAADF, SAXS), we will show that non-equilibrium bifurcation of phase transformation paths depending on alloy composition can be driven by the early stages of the decomposition of the solid solution.

9:50 AM  
Effect of Ca Additions on the Ageing Behavior of Mg-15Gd-0.5Zr Alloy: Houwen Chen1; Chenglong Liu1; Jian-Feng Nie2; 1Chongqing University; 2Monash University
    Mg-Gd-based alloys with Gd content more than 10 wt.% exhibit obvious precipitation hardening and high strength. The additions of elements, such as Zn and Ag, can enhance the age-hardening response of Mg-Gd binary alloys by the formation of additional precipitates on the basal plane of Mg matrix phase. In this presentation we will provide a detailed investigation of Ca additions on the ageing behavior of the Mg-15Gd-0.5Zr alloy. The results show that Ca additions slightly increase the age-hardening response of Mg-15Gd-0.5Zr alloy, which is attributed to the formation of basal precipitate plates with a crystal structure different from those reported in Zn- or Ag-containing Mg-Gd alloys. The basal precipitate plates co-exist with β′ phase in the peak-aged condition, and grow with ageing time. Ca atoms also segregate to β′ and β phases in the over-aged condition.

10:10 AM Break

10:30 AM  Invited
The Role of Electron Microscopy in the Understanding of Precipitation in Light Alloys: Jian-Feng Nie1; 1Monash University
    A common feature of precipitation hardenable aluminium and magnesium alloys is that their microstructures contain a uniform distribution of plate- or lath-shaped precipitates of intermediate or equilibrium phases, which form parallel to low-index planes of the matrix phase. It is well known that micro-alloying additions can change the identity and distribution of such precipitate phases. But there is still a lack of understanding of the precise role of micro-alloying elements in precipitate formation. This presentation will provide a review of observations made from conventional and modern transmission electron microscopy on strengthening precipitate phases in micro-alloyed aluminium and magnesium alloys. Discussions will be made on the correlation between these electron microscopy observations and our understanding of the roles of micro-alloying elements in precipitation in age-hardenable light alloys.

11:00 AM  
The Effects of ECAP on the Precipitation Behavior of Al 2024: Guher Tan1; Eren Kalay2; Hakan Gur2; 1Mersin University; 2METU
    Equal channel angular pressing (ECAP) is an important severe plastic deformation technique applicable to various alloys to improve the critical mechanical properties. Its application to Al 2024 alloy not only alters the precipitation hardening kinetics but also causes silver-free precipitation of omega phase which is well known for its good thermal stability. The current study investigates these behaviors in a single pass ECAPed Al 2024 alloy by means of advanced TEM analysis and microhardness measurements. The probable mechanisms underlying the 11 times faster kinetics of S precipitation at 190C and the reasoning behind the late formation of omega precipitates below 150C will be discussed in detail considering the microstructural variations in the deformed structure during thermal processing.

11:20 AM  
Analysis of Crystal Structures with Icosahedral Local Order in Al-Fe-V-Si Alloys After Solidification at Intermediate Cooling Rates: Joseph Jankowski1; Michael Kaufman; Amy Clarke; Stephen Midson; Krish Krishnamurthy2; 1Colorado School of Mines; 2Honeywell
    A two-phase powder of the h-AlFeVSi (P6/mmm a=2.522 nm c=1.256 nm) and α-AlFeVSi (Im-3 a=1.251 nm) icosahedral quasicrystalline approximants was prepared from a copper mold chill casting of RS8009 by dissolution of the aluminum matrix. The α-phase is a desirable strengthening phase in RS8009 while the h-phase is present as a coarse intermetallic. The crystal structure of the h-phase was solved from high resolution synchrotron powder diffraction using the charge flipping algorithm Superflip. Differences in local icosahedral order in the two phases will be presented. Additionally, the crystal structure solution from powder diffraction for the h-phase will be briefly compared to the previous model determined from TEM to highlight the advantages of high resolution powder diffraction.

11:40 AM  
Precipitate Structures in Mg Alloys Containing Nd and Y: Ellen Solomon1; Emmanuelle Marquis1; 1University of Michigan
    Magnesium rare earth (RE) alloys undergo significant strengthening during artificial aging due to the formation of solid-state precipitates with habit planes lying parallel to the prismatic planes. Although a lot of research studies have focused on precipitation in Mg-RE alloys, much work is still needed to fully characterize the physical nature of the precipitates to understand how they nucleate and grow. Therefore, this work focuses on quantifying the chemical and microstructural factors important in controlling phase stability and precipitation mechanisms in Mg-RE alloys by characterizing precipitates in model alloys including the Mg-Nd, Mg-Y, and Mg-Nd-Y systems using scanning transmission electron microscopy and atom probe tomography. The role of strains and chemistry will be discussed to explain the observed precipitate structures and their evolution during aging.